There is a need to be able to effectively control an internal combustion (IC) engine during initial engine operation, include starting, under a variety of unknown conditions, while meeting customer requirements for driveability and increasingly stringent government requirements for emissions performance. Conditions include use of fuels with a range of properties. The initial operation of the engine is also affected by various properties of intake air and the engine, including temperature, humidity, and remnants of fuel contained in the engine from previous operation. The fuel properties include vaporization pressure of the fuel, which is quantified using one of several indices, e.g. Reid Vapor Pressure (RVP) or Driveability Index (DI). Fuel refiners and distributors adjust fuel vaporization pressure to correspond to seasonal ambient temperatures in order to optimize cold start capability of IC engines in various geographic regions. The vaporization pressure is varied by shifting relative quantities of lower-, mid-, and heavier-weight hydrocarbon molecules contained in the fuel. The lower weight hydrocarbon molecules vaporize at lower temperatures, thus leading to more effective engine startability at low ambient temperatures. The fuel available may range in DI from under 1000 (highly volatile) in cooler areas to over 1250 (very stable) in hotter areas.
The fuel in a fuel tank may also change vaporization characteristics over time, through a process called ‘weathering’. The lower-weight hydrocarbon molecules may evaporate in the fuel tank. Passenger cars and trucks have evaporative systems that capture and store the evaporated hydrocarbons in a carbon canister and subsequently consume them by purging the canister through the engine. In engine applications wherein there is no evaporative system, the lower weight molecules may instead be vented to the atmosphere. In any event, the evaporative characteristics of the remaining fuel changes, along with the suitability of the fuel for cold start operation.
Fuel quality changes with variations in base oil stock from which the fuel is refined, and additives provided by the refiners to enhance fuel performance and boost octane levels. For example, refiners add various types of alcohol to boost oxygenate levels in fuels. There are also aftermarket additives that are used by vehicle operators and service technicians to boost performance or clean fuel system components. The use of additives may affect the evaporative characteristics of the fuel and the heat potential, or combustion energy, of the fuel.
The properties of intake air of concern include temperature, humidity, and any other sources of variations between actual air mass and measured air mass that affect fueling and combustion quality of the engine during initial operation. The properties of the engine of concern include engine component wear, engine temperature, presence of deposits on cylinder walls and elsewhere, and other properties that affect the ability of an engine to perform in the manner in which it was developed and calibrated.
Engine manufacturers also have customer driveability requirements for stable engine starting and operation. To meet the driveability requirements, engine management systems may be calibrated so a sufficient amount of fuel is delivered to make the system robust to varying levels of fuel volatility. A typical approach to managing varying levels of fuel volatility is to calibrate the system with excess fuel to ensure good driveability. Use of excess fuel during initial operation may have the effect of increasing engine-out hydrocarbon and carbon monoxide emissions. Engine manufacturers must also comply with more stringent exhaust emissions regulations in the future. An important strategy in meeting the emissions regulations is to ensure that the engine runs at an air/fuel ratio that is at or near stoichiometry at the start of the engine, or soon thereafter. The strategy is necessary to minimize engine out emissions and also to provide an exhaust gas feedstream to a catalytic converter that allows the converter to perform at optimum levels.
Engine and vehicle manufacturers accomplish the balance between meeting customer requirements for stable operation and meeting emissions regulations several ways. This includes conducting extensive pre-production testing and development to create and optimize engine operating calibrations, adding hardware, and increasing additional functionality to existing hardware. Extensive testing and calibration is conducted during the engine development phase. Hardware such as air injection pumps may be added to assist in oxidizing engine emissions during initial engine operation, which includes engine start. The amount of precious metals (palladium, rhodium, and platinum) contained in the catalytic converter may be increased to improve effective conversion of unwanted emissions. Each of these methods adds complexity and cost to the vehicle or engine.
Several methods have been proposed to control engine performance based upon fuel quality and volatility by monitoring the engine during initial operation. These methods infer volatility from other measured parameters, including engine speed, cylinder pressure ratio, or exhaust gas temperature measurement. Examples of these methods are described in U.S. Pat. No. 6,283,102, entitled Fuel Identifier Algorithm, issued to Nelson on Sep. 4, 2001, U.S. Pat. No. 6,178,949, entitled Engine Control Having Fuel Volatility Compensation, issued to Kirwan on Jan. 30, 2001, and U.S. Pat. No. 5,875,759, entitled Method for Improving Spark Ignited Internal Combustion Engine Starting And Idling Using Poor Driveability Fuels, issued to Meyer on Mar. 2, 1999.
Each of these methods carries the disadvantage that they do not directly measure or compensate for variations in combustion quality, especially as affected by fuel volatility and fuel quality. Compensating for variation in combustion quality is especially important during initial operation of the engine, including a cold start. Therefore any compensation scheme may be skewed because of incorrect assumptions in the inference chain from the measured parameter to a useable parameter, i.e. combustion quality. Each method also requires varying levels of testing and evaluation during engine calibration and development to establish the inference chains and create calibration tables that may be used by an engine controller. Each method also may have to be regularly reset to a nominal value during the operation of the vehicle due to external changes for which the given method is unable to adjust, e.g. vehicle refueling with a different volatility of fuel. Each of the methods also carries the disadvantage that they do not provide compensation for fuel quality and volatility. Furthermore, each method is unable to compensate for performance variations between cylinders.